prop length

[Captain Avgas]

The 72" vs 74" debate has been extensively covered in the archives. Probably the best thread is this one: http://www.vansairforce.com/communit...ht=prop+length

In Post #6 of that thread gmcjetpilot (good ol' George) makes a convincing argument based on mathematics and discussions with Hartzell that a 72" prop may provide a VERY SLIGHT top speed advantage and that a 74" prop may provide a VERY SLIGHT climb advantage. But a full reading of the post should convince anyone that the advantages either way are truly miniscule and are not worth worrying about in the real world (unless maybe you plan to race at Reno and desperately need half a knot....in which case you'd opt for the 72").

I would propose therefore that the only real difference between the Hartzell 72" prop and the 74" prop is their propensity for a ground strike....and in that category the 72" prop with its greater ground clearance clearly wins.

But there will always be builders out there who believe that at the same price the 74" prop gives you more metal and therefore must be the better bargain. I guess it's the bigger must be better (size really counts) philosophy.

I was a passenger in an RV that suffered a prop strike a couple of weeks ago. The strike caused the crank shaft to completely shear off and the prop departed the aircraft taking the starter ring gear with it. Needless to say the prop and the engine (only 100 hours since brand new) were totally decimated. If I wasn't a believer already that was enough to convince me that CLEARANCE IS KING. It's like money...you just can't have enough of it.

[elippse]

Originally Posted by hevansrv7a

Please see embedded questions. Not arguing, seeking to understand.

The same is true for multi-blade props because for a given diameter, a three-blade prop will have 50% more mass flow. (Doesn't this assume equal blades area, shape, CL, etc., which would not be feasible?), so less energy gets thrown away in downwash.(But aren't longer blades better for induced loss, if all else is equal?)

Mass-flow should not be confused with blade area. Lift, or thrust, is the product of area, Q, and CL. If you have more blades, you reduce the area on each blade by the ratio of the number of blades, but you keep the CL the same, so the overall thrust is the same. Induced loss is a function of mass-flow which has to do with disc area, which is what was stated in the first sentence. If you keep the diameter the same and increase the number of blades the mass flow goes up, the downwash, which has to do with induced loss, goes down, and the efficiency, at least at low speed, goes up. The latter is true since at high speed the delta-v only contributes a very small portion to the mass flow since most of it comes from forward speed. That is why a multi-blade prop has better static thrust and its attendant better take-off and climb, where speed and mass flow are low.
It's unfortunate that in the aviation community the mis-conception that multi-blade props aren't as good in cruise as a prop with fewer blades was based on the fact that the aerodynamic shape of the blade roots at that time contributed so much drag to the overall plane's drag; more blades, more drag. Unfortunately, the "experts" attributed this to some mythical "tip loss" fuction, and every one nodded their heads in affirmative unison and repeated "I see, I see!".
Even with Tom Aberle dominating the biplane races with a four-blade prop, the skeptics still attribute this to enough engine power to overcome the supposed multi-blade drawback. Just look at the 8-blade props on the A-400M, the same on the new C-131J, and the 18 to 24 blade fixed pitch fan (prop) in the front of a jet engine. Are these designers laboring under some delusion? It is time for the aviation community to put this misunderstanding to rest!

[Ron Lee]

Originally Posted by Captain Avgas

I was a passenger in an RV that suffered a prop strike a couple of weeks ago. The strike caused the crank shaft to completely shear off and the prop departed the aircraft taking the starter ring gear with it. Needless to say the prop and the engine (only 100 hours since brand new) were totally decimated. If I wasn't a believer already that was enough to convince me that CLEARANCE IS KING. It's like money...you just can't have enough of it.

Was that in a taildragger?

[Captain Avgas]

Originally Posted by Ron Lee

Was that in a taildragger?

It was an RV6A.

[Kevin Horton]

Originally Posted by Captain Avgas

It was an RV6A.

Tell us more. What were the circumstances that lead to the prop strike?

[Captain Avgas]

Originally Posted by Kevin Horton

Tell us more. What were the circumstances that lead to the prop strike?

Kevin, it's not my plane. I was just a passenger. The damage is the subject of a pending insurance claim by the aircraft's owner and therefore it is not appropriate for me to be making any public comments about the circumstances.

The point I was trying to make however is that a prop strike, for whatever reason, can cause very serious damage to the engine and is therefore best avoided.

[MikeS]

A person has to really work to hit the prop on a wheel landing. Maybe with a VERY hard drop or a deliberate dive into the runway as some newbies do when trying to force a bouncing plane on the ground. Langwiesche correctly illustrates in S&R that airflow over the horizontal stabilizer will counter any attempt to hit the prop when doing a wheel landing. Once slowed down and with less flow over the stabilizer I suppose one could tip the plane over and hit the prop but that would take either some very deliberate intention or some extreme ineptitude.

I once had, with a KR-2 taildragger, about 4.5 inches of clearance in the level position and I was constantly worried about hitting the prop when wheel landing. It was a Maloof CS metal prop - quite pricey and not as forgiving as wood. This led to quite a lot of apprehension and less than graceful landings when I first got the plane.

Re-reading Langewiesche's chapter on Landings, I decided to trust his wisdom as I was tired of worrying about a prop strike. Many years later I continue to fly a taildragger with not a great deal of clearance in the level position and never worry in the slightest about a runway strike. Langewiesche is correct. Relax about prop strikes when landing. It can be done, but you really need to screw up badly to do it.